Here's some additional info from the FAQ section of a Dr. Pepper site on the internet:

What is the meaning of "10-2-4"?An early study on human fatigue showed that human energy dropped to its lowest levels during the day at 10:30 AM, 2:30 PM, and 4:30 PM daily. Therefore, drinking Dr Pepper at 10 AM, 2 PM, and 4 PM would boost energy. Thus the "10-2-4" symbol!